I have observed a number of people who have completely changed their lifestyle from irreligious to observant (whom we now call “baalei teshuvah”) standing for all the davenning and even for the drosho.

I asked (sensitively) one person who stands for everything why he does it. He answered that he is mostly tired. Standing helps him stay awake. I wonder, is this modesty?
–
Avrohom YitzchokDec 25 '12 at 17:55

1

No it's not modesty. As a person who stands through all of shul, I do it so I don't fall asleep or have my mind wander.
–
aviFeb 3 '13 at 19:53

While I don't stand for everything, I can say that it helps me to pray more quickly. So if I am likely to fall behind during Psukei D'Zimrah, I will often stand. Baalei teshuva are likely to be more prone to falling behind and wanting to keep pace. It could be that it is a habit learned early in increasing observance which then sticks. At least, this has been roughly my own experience.

More likely than being based on a source or a recommendation, is the fact that most Baal Tshuva people did not grow up going to Shul and sitting through services for such a long amount of time. They are therefore more likely to start falling asleep or having their mind wander and standing up prevents that from happening by being mildly uncomfortable. (Or atleast they think they are more likely, and don't have childhood practice to know differently)